A great variety of techniques are being used for the synthesis of nanoparticles of inorganic compounds. Most of these techniques suffer from lack of precision in controlling particle size and properties. The current state of the art in the synthesis of semiconductor nanocrystals involves the use of high temperature batch reactors. This process uses a hot coordinating solvent, such as hexadecylamine and trioctyl-phosphine, in which the reactants are injected with a syringe. Particles grow as a function of time and samples are taken at specific times to obtain populations of a certain average size. It is difficult to control particle size distributions in such reactors and almost impossible to isolate particles with a specific, pre-determined particle size. Using this approach, post-processing and functionalization requires many additional steps that can compromise the quality of the particles. Further, the technique cannot be scaled-up easily for industrial production.
Semiconductor nanocrystals (quantum dots) are exotic materials whose optical and electronic properties can be manipulated by changing their size or composition (Alivisatos, Science 271:933 (1996); Murray et al., Annu. Rev. Mater. Sci. 30:545 (2000)). When the size of the nanocrystals becomes smaller than the corresponding de Broglie wavelength or Bohr radius (mean separation of an optically excited electron-hole pair), quantum confinement phenomena take place and change the nanocrystal properties dramatically. II-VI quantum dots (e.g., CdSe, CdS, ZnS, or ZnSe), with sizes of a few nanometers (“nm”), exhibit size-dependent luminescence, broad excitation by all wavelengths smaller than the emission wavelength, high brightness, narrow and symmetric emission, and excellent photostability (Alivisatos, Science 271:933 (1996); Murray et al., Annu. Rev. Mater. Sci. 30:545 (2000)). In addition to playing an important role in fundamental studies on solid-state physics (Empedocles et al., Adv. Mater. 11:1243 (1999)), quantum dots can be used in photovoltaic devices (Huynh et al., Adv. Mater. 11:923 (1999)), photodetectors (Towe et al., IEEE J. Sel. Top. Quant. Electr. 6:408 (2000)), and as fluorescent biological labels (Michalet et al., Single Mol. 2:261 (2001)).
The most common synthesis route for II-VI nanocrystals involves reactions between organometallic compounds in a trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) and/or hexadecylamine (HDA) coordination solvent carried out in small batch reactors operating at ˜300° C. CdSe and CdS quantum dots have been the most common materials grown by this technique (Murray et al., J. Am. Chem. Soc. 115:8706 (1993)). Luminescent ZnSe nanocrystals exhibiting high quantum yield (Hines et al., J. Phys. Chem. B. 102:3655 (1998); Revaprasadu et al., J. Mater. Chem. 8:1885 (1998)) and (Zn,Mn)Se diluted magnetic nanocrystals (Norris et al., Nano Lett. 1:3 (2001)) have also been grown. To grow monodisperse nanocrystal populations the requirements include instantaneous injection and mixing of the reactants, uniform nucleation over the entire mass of the solvent, and perfect mixing during the entire process. Such conditions are difficult to achieve and selective precipitation techniques are used after synthesis to narrow down the size distribution of the nanocrystals (Murray et al., Annu. Rev. Mater. Sci. 30:545 (2000)). Other reported techniques for growing ZnSe nanocrystals include arrested precipitation (Chestnoy et al., J. Chem. Phys. 85:2237 (1986)), sol-gel processing (Li et al., J. Appl. Phys. 75:4276 (1994)), sono-chemical processing (Zhu et al., Chem. Mater. 12:73 (2000)), growth in reverse micelles (Quinlan et al., Langmuir 16:4049 (2000)), and vapor-phase synthesis (Sarigiannis et al., Appl. Phys. Lett. 80:4024 (2002)).
The use of a template is typically required for growing monodisperse particle populations. Control of particle microstructure has been achieved by colloidal crystallization in aqueous droplets suspended on the surface of a fluorinated oil (Velev et al., Science 287:2240 (2000)). Monodisperse populations of Si quantum dots, with surfaces passivated by an organic monolayer, were grown by thermally degrading diphenysilane in supercritical octanol (Holmes et al., J. Am. Chem. Soc. 123:3743 (2001)). ZnSe nanocrystals were grown in bis-2-ethylhexylsulphosuccinate sodium salt (AOT) reverse micelles by reacting zinc perchlorate hexahydrate and sodium selenide (Quinlan et al., Langmuir 16:4049 (2000)). Under ideal conditions, reverse micelles could function as identical nanoreactors, thus providing a template for precise control of particle size. In practice, the fast dynamics of droplet coalescence in water-in-oil microemulsions lead to the formation of droplet clusters and polydisperse particle populations (Zhao et al., Langmuir 17:8428 (2001)).
The present invention is directed to overcoming these and other deficiencies in the art.